newcomer to TI DDK

I am now working on TI Dm642 evm and studying DDK. I have some problems
with the video port driver.

in mdBindDev, it calls mdControlChan, as follows,
===================

static Int mdBindDev(Ptr *devp, Int devid, Ptr devParams)
{
    Int portNum = devid;         
    volatile Int i;
    volatile Int* base = (volatile Int *)portObjs[portNum].base;    
    assert(portNum < _VP_PORT_CNT);
    base[_VP_VPCTL_OFFSET] = VP_VPCTL_VPRST_RESET <<
_VP_VPCTL_VPRST_SHIFT;
    for(i = 0; i < 100000; i ++);             
   
    *devp = &portObjs[portNum];                          
    return mdControlChan(&portObjs[portNum], VPORT_CMD_CONFIG_PORT,
devParams); 
}
=======================
but why in mdControlChan, we can typecast the (pointer to PortObj) to
(pointer to _VPORT_ChanObj)?

static Int mdControlChan(Ptr chanp, Uns cmd, Ptr args)
{
    Int retVal = IOM_EBADMODE;
    _VPORT_ChanObj* chan = (_VPORT_ChanObj* )chanp;
    PortObj* port = &portObjs[chan->portNum];
....

==========================

Can anyone help me? I am really appreciate your help.

hmq wrote:
> 
> in mdBindDev, it calls mdControlChan, as follows,
...
> but why in mdControlChan, we can typecast the (pointer to PortObj) to
> (pointer to _VPORT_ChanObj)?
...

Notice that in mdBindDev, the call to mdControlChan has
VPORT_CMD_CONFIG_PORT as the second parameter. Looking at mdControlChan,
the switch statement contains:

...
    case VPORT_CMD_CONFIG_PORT:
        retVal = _configPort(chanp, args);
        break;
...

Now, looking at _configPort, we see that:

static Int _configPort(Ptr portp, Ptr args)
{
    PortObj* port = (PortObj *)portp;
....

So, calling mdControlChan with VPORT_CMD_CONFIG_PORT is a special case.
In this case, chanp is a pointer to a port (PortObj *), in all other
cases it's a pointer to a channel (_VFVPORT_ChanObj*).

Cheers

mark-r

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